Overprinting is the intentional printing of one or more layers of ink, or other such printing media, on top of another, for example by coating or varnishing. In this manner, a previously printed media can be enhanced, for example, with a glossy finish. The overprint or overcoat can be on the entire media or substrate, on one or more features previously printed on the substrate or be printing printed adjacent to previously printed features, or any combination of these.
To print on specific features and produce a high quality overprint, it is critical that the overprint ink be accurately applied on the substrate. For this purpose, the overprint coating applicator, for example, an ink-jet nozzle, must be accurately aligned with the features to be coated.
Another example of overprinting one in which a symbol or text, etc, is printed after a first printing, and the overprinting is printed adjacent or in proximity to, but not necessarily directly over, the symbol or text.
U.S. Pat. No. 4,857,715 (Koch et al) discloses a scannable form for an optical mark scanning apparatus in the form of a generally rectangular sheet of paper or like material having a preprinted timing track along one edge and a plurality of preprinted quality assurance marks is printed by a laser printer with customized questions and corresponding response bubbles to create a survey form. An overprint registration system is used in conjunction with the scannable forms to align material for printing on the scannable forms prior to printing the survey form by printing an overprint registration mark corresponding to at least one of the quality assurance marks and adjusting the position of the overprint registration mark to establish the alignment between the response bubbles to be printed and the preprinted timing track. The system may also include a verification process wherein a plurality of alignment marks will be overprinted in the position of the overprint registration marks during the printing of the survey form, so that the alignment of each form in relation to the quality assurance marks may be verified during scanning.
U.S. Pat. No. 5,600,350 (Cobbs et al) describes an image registration system for a multicolor inkjet printer/plotter. The system comprises a carriage assembly for retaining multiple inkjet cartridges. Each cartridge has a plurality of nozzles adapted to eject ink in response to the application of an electrical signal thereto. A first mechanism is provided for moving the carriage assembly means in a first axis. A second mechanism is provided for moving print media in a second axis transverse to the first axis, the first axis being a scan axis and the second axis being a media axis. A first position encoder senses the position of the carriage assembly in the first axis and a second position encoder senses the carriage assembly in the second axis and providing position encoder signals in response thereto. A control circuit provides electrical signals which cause the nozzles in the inkjet cartridges to eject ink onto the media and create an image thereon in response to timing signals. The system includes a sensor module which optically senses the image and provides a set of sensed signals in response thereto. The sensed signals are processed to provide timing signals for use in correcting the image miss-registration.
U.S. Pat. No. 6,454,383 (Lund et al) provides a method and apparatus for a test pattern used in the alignment of an ink-jet pen which deposits fixer fluid, or other clear ink precursor fluid, on print media uses the change in reflectivity caused by overprinting a series of positional-calibration indicia with colorant to obtain data with respect to deviations in a carriage-scan x-axis and a paper scan y-axis. Thus the invention measures distances between pens or nozzles.
U.S. Pat. No. 5,803,504 (Deshiens et al) describes a method of producing a lottery ticket with an overprint region provided over a scratch-off layer. To ensure proper alignment of printing layers of the overprint region, photocell devices are installed at each of the stations of press maybe linked to various controls of the paper feeding mechanism of the press. Preferably, the registration devices in the flexographic press should allow no more than a 0.005 inch (0.013 cm) variation on each station.
U.S. Pat. No. 6,840,173 (Kawabata et al) discloses a plate cylinder and printing plate holder for the cylinder, which is capable of adjusting relative positions of plural printing plate holders and fixing the printing plate holders on the plate cylinder without causing misalignments of images among printing plates held by the printing plate holders even after overprinting. The plate cylinder is equipped with at least two printing plates wrapped around the outer circumference thereof. The plate cylinder comprises printing plate holders, one per each printing plate, for holding the printing plates on the outer circumference of the plate cylinder. The printing plate holders include one printing plate holder in a stationary state and fixed against the plate cylinder and other printing plate holders that are adjustable to move in the circumferential direction of the plate cylinder and fixable against the plate cylinder.
U.S. Pat. No. 5,434,956 (Myungsea son et al) discloses a method and apparatus for printing an image in a specified positional relationship with a preprinted registration mark. Each two-dimensional alignment mark on a printing medium is found by using information about the whole mark area, or with respect to each of two different directions relative to the medium by using a distinctive two-dimensional pattern or characteristic of the mark; or by finding the mark centroid, or by scanning the mark region to obtain a two-dimensional representation for analysis—and preferably by combinations of these procedures. An image is then aligned and printed by reference to the mark so located. A previous image may also be preprinted in known relation with the mark, so that the new image is aligned with the preprinted one—sometimes on the same piece of medium. Preferably information used includes the mark's intended size, shape, areal disposition and other properties; and a two-dimensional search template is defined which matches the mark in at least one of these. A template position is then found that essentially maximizes areal intersection with the mark; this position is then treated as the mark position, for printing the new image. The invention preferably determines a position of the template along each of two directions in alternation—maximizing the areal intersection with respect to shifting along each direction, iteratively—until no significant increase is obtained.
U.S. Pat. No. 6,325,480 (Moghadam et al) discloses an inkjet printer and method capable of forming a plurality of registration marks on a receiver and sensing the marks formed thereby. The method includes a print head for printing an image of predetermined length on the receiver. The receiver has an image area for receiving the image therein and a border area adjacent to the image area. A marker forms the plurality of registration marks in the border area, so that the marks extend the length of the image. In addition, a sensor is disposed in sensing relationship to the marks for sensing the marks. The invention provides a combination marker for marking a receiver and a sensor for sensing the marks so that each image line is in registration with other lines of the image. Also, use of the invention avoids need for costly precision motors to advance the receiver during printing of image lines.
To aid in applying the overprint coating accurately, prior art relies on registration markers, typically in the form of cross hairs, located in two corners of the substrate. However, during the pre-overprinting print, registration markers and the specific features may be misaligned; i.e. moved from their intended location in an x-direction, and/or a y-direction, and/or at an angle, and/or due to scaling (enlargement/reduction) and so on. Other types of misalignment can also be present, as will be discussed in more detail below.
It is therefore a long felt need to disclose a means and method for overprinting that overcomes the difficulty of identifying a misalignment between registration markers and preprinted features, such that the overprint is not aligned in accordance with said registration markers, but rather in accordance with said pre-printed features. Moreover, it is a long felt need to address a plurality of printing shifts beyond misalignment, namely global shift, a local shift, a linear shift, an angular shift, a size shift, an intensity shift, a color shift, or any combination thereof.
It is also a long felt need to compensate for the misalignment and shifts using image processing rather than by mechanical means (a set of signals to the printer), thus enabling to address any printer suitable for the task.